Around 2.4 billion years ago, the ancestors of bacteria took over the oceans and began photosynthesizing, creating massive amounts of oxygen where before there had been barely any. This was the Great Oxygenation Event, which made all subsequent life on Earth possible. All life except . . . yeast.
2.5 billion years ago, some highly-efficient bacteria began what's known as the oxygen revolution. 600 million years ago, the first multicellular animals appeared. What happened in between? We're really not sure - but geologists are working on it.